WO2024024533A1

WO2024024533A1 - Bone screw

Info

Publication number: WO2024024533A1
Application number: PCT/JP2023/025979
Authority: WO
Inventors: 康平森田; 洋輝大橋; 諭谷; 雄一村山; 健司土田; 泰浩古田
Original assignee: 株式会社スパインテック; 株式会社タカイコーポレーション
Priority date: 2022-07-29
Filing date: 2023-07-14
Publication date: 2024-02-01

Abstract

In order to provide a bone screw having adequate strength, a bone screw (1) includes reinforcing fibers (100) and comprises a shaft portion (2), a tip end portion (3) located at an end of the shaft portion in an insertion direction, a head portion (4) located at an end of the shaft portion in a removal direction, and a screw thread (5) located with a helical shape in an axial direction of the shaft portion on an outer circumferential portion of the shaft portion, wherein the reinforcing fibers are configured to be disposed in the shaft portion in locations extending in the axial direction of the shaft portion.

Description

bone screw

The present invention relates to bone screws that can be fixed to animal bones.

In order to fix the bones of animals (particularly humans) in surgeries in fields such as neurosurgery, bone screws have been developed that include a tip that pierces the bones (see Patent Document 1).

Conventionally, bone screws made almost entirely of metal (such as stainless steel) or PEEK (registered trademark) have been proposed in order to maintain the strength of fixing bones.

Patent No. 5443398

However, a bone screw made almost entirely of metal (stainless steel, etc.) causes artifacts when taking medical images, and becomes an obstacle when performing surgery, etc. while checking the medical images. Furthermore, PEEK (registered trademark) may not have sufficient strength.

The bone screw of the present invention is a bone screw containing reinforcing fibers, and includes a shaft portion, a tip portion located at the end of the shaft portion in the insertion direction, and a head portion located at the end of the shaft portion in the withdrawal direction. and a screw thread located spirally in the axial direction of the shaft part on the outer circumferential part of the shaft part, and the reinforcing fiber is arranged in the shaft part at a position extending in the axial direction of the shaft part. be done.

According to the present invention, a bone screw with sufficient strength can be provided.

They are a side view and a sectional view of the bone screw of this embodiment. It is a sectional view of a shaft part of a bone screw of this embodiment, and a screw thread. It is a figure which shows that the angle of the thread of this embodiment becomes large within the range of 60 thru|or 80 degrees, so that it goes to the insertion direction of a bone screw. FIG. 4(a) is a diagram showing the head of the bone screw of this embodiment. FIG. 4(b) is a sectional view taken along line BB of the bone screw of this embodiment. It is a perspective view of the bone screw of this embodiment. It is a perspective view of the bone screw of this embodiment. FIG. 2 is a sectional view taken along line CC of the bone screw of the present embodiment. It is a figure which shows the example of the thread type of the bone screw of this embodiment. It is a figure which shows the other example of the thread type of the bone screw of this embodiment. It is a figure which shows that the bone screw of this embodiment accommodated in the housing is fixed by the nut in the housing, and is connected by the rod. This is a CT image (medical image) taken with a bone screw 1 screwed into an animal bone. 2 is a table that evaluates bone screw artifacts in medical images taken according to tantalum content.

The bone screw of this embodiment will be explained using the drawings. FIG. 1 is a side view and a sectional view of the bone screw of this embodiment. FIG. 2 is a cross-sectional view of the shaft portion and screw thread of the bone screw of this embodiment.

As shown in FIG. 1(a), the bone screw 1 of this embodiment is a bone screw containing reinforcing fibers, and includes a shaft portion 2 and a tip portion 3 located at the end of the shaft portion 2 in the insertion direction. , a head 4 located at the end of the shaft portion 2 in the removal direction, and a screw thread 5 located spirally in the axial direction of the shaft portion 2 on the outer circumference of the shaft portion 2.

FIG. 1(b) is a cross-sectional view taken along line AA in FIG. 1(a). As shown in FIG. 1(b), the reinforcing fibers 100 are arranged in the shaft portion 2 at a position extending in the axial direction of the shaft portion 2. FIG. 1(c) is an enlarged cross-sectional view of the thread 5 of FIG. 1(b). As shown in FIG. 1(c), the reinforcing fiber 100 is bent at the thread 5.

Further, although not shown, the reinforcing fibers 100 are arranged in the thread 5 at a position extending in the same direction as the helical direction of the thread 5. The reinforcing fibers 100 are arranged at positions extending along the helical direction of the threads 5.

Further, although not shown, the reinforcing fiber 100 is spirally wound in the axial direction around at least one of the shaft portion 2 and the screw thread 5.

When injection molding a bone screw using the reinforcing fiber 100, a mixed composition of the reinforcing fiber 100 (for example, 1 mm carbon fiber) and a resin (for example, vinyl ester) is placed in the mold of the bone screw 1, and the bone It is ejected from the head side of the screw 1 in the insertion direction of the bone screw 1. In this case, when forming the mixed composition, the reinforcing fibers 100 are mixed with the resin so that the reinforcing fibers 100 are arranged in a predetermined direction. For example, the reinforcing fibers 100 are mixed with the resin so that the reinforcing fibers 100 are arranged in the axial direction of the bone screw 1. Alternatively, the reinforcing fibers 100 are mixed with the resin so that the reinforcing fibers 100 are arranged at a position extending at a predetermined angle with respect to the axial direction of the bone screw 1. Alternatively, the reinforcing fibers 100 are mixed into the resin so that they are arranged in the helical direction of the threads 5 of the bone screw 1. The resin includes at least one of a thermosetting resin and a thermoplastic resin. For example, the resin may be selected from epoxy resins, phenolic resins, unsaturated polyester resins, and the like.

As described above, by injecting a mixed composition in which the reinforcing fibers 100 are mixed into a predetermined position into the mold of the bone screw 1, the reinforcing fibers 100 are stretched in the axial direction of the shaft part 2 in the shaft part 2. The reinforcing fiber 100 may be bent at the thread 5 or may be arranged at a position extending in the same direction as the helical direction of the thread 5, or the reinforcing fiber 100 may be bent at the thread 5, or may be arranged at a position where it extends in the same direction as the helical direction of the thread 5. , and may be arranged in a position where it is wound spirally around the axial direction.

In the molded bone screw 1, the reinforcing fibers 100 are appropriately arranged in a predetermined direction to strengthen at least one of compressive strength, tensile strength, shear strength, torsional strength, and hardness of the bone screw 1. Can be done.

The reinforcing fiber 100 is preferably at least one of carbon fiber, aramid fiber, and glass fiber, but is not limited to these.

FIG. 2 is a cross-sectional view of a bone screw 1 in which a mixed composition of reinforcing fibers 100 and a resin is injection molded into a mold for the bone screw 1. The angle of the thread 5 is 60 degrees or more and 80 degrees or less.

As a result of extensive research, the inventor injected a mixed composition of reinforcing fibers 100 and resin into a mold for the bone screw 1, and determined the angle of the thread 5 for appropriately molding the thread 5 of the bone screw 1. I found out. If the angle of the threads in the mold is too small, the mixed composition will not be sufficiently injected to the threads, and the threads 5 will not be properly molded. On the other hand, if the angle of the threaded portion of the mold is too large, the fixing strength of the bone screw 1 to the bone will be insufficient and it will easily come off from the bone.

In FIGS. 2(a) and 2(b), the angles of the threads 5 are 120 degrees and 100 degrees, respectively. As shown in FIGS. 2(a) and 2(b), although the mixed composition is sufficiently injected to the threaded portion, the angle of the threaded portion is too large and easily detaches from the bone.

In FIGS. 2(c) and 2(d), the angles of the threads 5 are 80 degrees and 60 degrees, respectively. As shown in FIGS. 2(c) and 2(d), the mixed composition is sufficiently injected to the threaded portion, and the angle of the threaded portion is not too large and does not easily detach from the bone. Therefore, it is appropriate that the angle of the thread 5 is 60 degrees or more and 80 degrees or less. Preferably, the angle of the thread 5 is 60 degrees or more and 75 degrees or less. More preferably, the angle of the thread 5 is 65 degrees or more and 70 degrees or less.

FIG. 3 is a diagram showing that the angle of the thread 5 increases in the range of 60 degrees to 80 degrees as it goes toward the insertion direction of the bone screw 1. As shown in FIG. 3, the angles θ1 to θ3 of the screw threads 5 increase in the range of 60 degrees to 80 degrees as the direction of insertion of the bone screw 1 increases (θ1<θ2<θ3). Preferably, the angle of the thread 5 increases in the range of 60 degrees or more and 75 degrees or less as it goes toward the insertion direction of the bone screw 1. More preferably, the angle of the thread 5 increases in the range of 65 degrees or more and 70 degrees or less as it goes toward the insertion direction of the bone screw 1. The angle of the thread 5 may gradually increase within the above range or may increase intermittently as it goes in the insertion direction of the bone screw 1, or a part of the thread 5 may It may become larger within the above range as it goes in the insertion direction of the screw 1. Further, the heights h1 to h3 of the screw threads 5 may become lower toward the insertion direction of the bone screw 1. The heights h1 to h3 of the threads 5 may vary depending on the angle of the threads 5.

FIG. 4(a) is a diagram showing the head 4 of the bone screw 1. FIG. 4(b) is a sectional view of the bone screw 1 taken along line BB. As shown in FIG. 4(b), the head 4 includes a head protrusion 41 in the insertion direction, and the head protrusion 41 fits into the shaft recess 21 of the shaft portion 2. Furthermore, the joint between the head 4 and the shaft portion 2 (the convex head portion 41) has higher at least one of compressive strength, tensile strength, shear strength, torsional strength, and hardness than the shaft portion 2. For example, the head convex portion 41 is made of at least metal (e.g., titanium, stainless steel, iron, nickel, cobalt, cemented carbide, etc.), ceramic (e.g., zirconia), cermet, and reinforcing fibers having a composition different from that of the reinforcing fibers 100. Contains one. Here, the reinforcing fibers having a different composition from the reinforcing fibers 100 may be reinforcing fibers of the same type as the reinforcing fibers 100, as long as at least one of the mixing ratio, length, shape, and arrangement of the reinforcing fibers 100 is different. Furthermore, the reinforcing fibers having a different composition from the reinforcing fibers 100 may be reinforcing fibers of the same type as the reinforcing fibers 100 if other reinforcing fibers are mixed therein.

Note that even without the head protrusion 41 and the shaft recess 21, the joint between the head 4 and the shaft portion 2 has higher compressive strength, tensile strength, shear strength, torsional strength, and hardness than the shaft portion 2. It is sufficient that at least one of them is high.

For example, although not shown, the shaft portion 2 may include a shaft protrusion in the removal direction, and the shaft protrusion may fit into a head recess of the head 4. That is, in FIG. 4, the head 4 is provided with a convex portion and the shaft portion 2 is provided with a concave portion, but conversely, the shaft portion 2 may be provided with a convex portion and the head 4 may be provided with a concave portion. Although not shown, at least one of metals (e.g., titanium, stainless steel, iron, nickel, cobalt, cemented carbide, etc.), ceramics (e.g., zirconia), cermet, and reinforcing fibers having a composition different from that of the reinforcing fibers 100 The core part may be fitted into a head recess of the head 4 and a shaft recess of the shaft part 2. That is, the head 4 and the shaft portion 2 are provided with recesses, and the recesses are connected by the core. In addition to these, a portion having at least one of compressive strength, tensile strength, shear strength, torsional strength, and hardness higher than that of the shaft portion 2 may be provided at the joint portion between the head portion 4 and the shaft portion 2. . In this case, the joint may be a boundary between the head 4 and the shaft 2 made of different materials, as shown in FIG. There does not have to be any boundary surface. For example, if at least one of the mixing ratio, length, shape, and arrangement of the same materials (e.g., the same reinforcing fibers) are continuously different, there is no clear boundary surface, but the compressive strength is higher than that of the shaft part 2. , a portion having high at least one of tensile strength, shear strength, torsional strength, and hardness may be provided between the head 4 and the shaft portion 2.

As a result, at least one of compressive strength, tensile strength, shear strength, torsional strength, and hardness can be strengthened in the part between the head 4 and the shaft part 2 where the load is applied, and the bone screw 1 can be broken. and deformation can be prevented.

FIG. 5 is a perspective view of the bone screw 1. FIG. 6 is a perspective view of the bone screw 1 of FIG. 5. FIG. 7 is a sectional view taken along line CC of the bone screw 1 in FIG.

As shown in FIGS. 5 to 7, the head 4 includes a head protrusion 41 in the insertion direction, and the head protrusion 41 fits into the shaft recess 21 of the shaft portion 2.

Further, as shown in FIG. 6, the head convex portion 41 includes a polygonal portion 41-1 whose cross section perpendicular to the axial direction is a polygon. The polygonal portion 41-1 has a substantially rectangular cross section perpendicular to the axial direction, and is chamfered. The polygonal portion 41-1 is preferably approximately triangular to approximately hexagonal. Since the head protrusion 41 has a polygonal cross-section perpendicular to the axial direction, when torque is generated in the rotational direction of the bone screw 1, the head protrusion 41 slides or comes off from the shaft recess 21 in the rotational direction. This can be prevented.

As shown in FIG. 7, the head protrusion 41 includes a notch 22. The cutout portion 22 may include a rotation surface in the rotational direction of the bone screw 1, or may have a flange shape. By providing the head protrusion 41 with the notch 22, it is possible to prevent the head protrusion 41 from slipping or coming off from the shaft recess 21 in the axial direction. Further, although not shown, the head convex portion 41 may include a screw thread located in a reverse helical direction to the screw thread 5.

Similarly, the shaft recess 21 may include at least one of a polygonal section whose cross section perpendicular to the axial direction is a polygon, a thread located in a reverse helical direction with respect to the thread 5, and a notch.

In addition, as described above, when the convex part of the shaft part 2 fits into the recessed part of the head part 4, or when the core part fits into the head recessed part of the head part 4 and the shaft recessed part of the shaft part 2, The convex part of the shaft part 2, the concave part of the head part 4, and the core part have a polygonal part whose cross section perpendicular to the axial direction is a polygon, or a thread located in a reverse spiral direction with respect to the thread 5, and a notch part. At least one may be provided.

Note that the core portion may include reinforcing fibers arranged at at least one of a position extending in the axial direction and a position spirally wound around the axial direction.

As shown in FIGS. 6 and 7, the bone screw 1 includes a hole 20 that can pass through the inside of the bone screw 1 in the axial direction. 6 and 7, the hole 20 passes through the interior of the bone screw 1 from the recess 40 of the head 4 to the distal end 3. In FIGS.

The hole 20 is made of at least one of metal (e.g., titanium, stainless steel, iron, nickel, cobalt, cemented carbide, etc.), ceramic (e.g., zirconia), cermet, and reinforcing fiber having a composition different from that of the reinforcing fiber 100. Surrounded by containing materials. Here, the reinforcing fibers having a different composition from the reinforcing fibers 100 may be reinforcing fibers of the same type as the reinforcing fibers 100, as long as at least one of the mixing ratio, length, shape, and arrangement of the reinforcing fibers 100 is different. Furthermore, the reinforcing fibers having a different composition from the reinforcing fibers 100 may be reinforcing fibers of the same type as the reinforcing fibers 100 if other reinforcing fibers are mixed therein.

Hole 20 surrounded by a material including metal (e.g., titanium, stainless steel, iron, nickel, cobalt, and cemented carbide), ceramic (e.g., zirconia), cermet, and reinforcing fibers having a composition different from reinforcing fiber 100 When the image is captured in a medical image such as a CT image with higher brightness than the surrounding area of the hole 20, the hole 20 functions as a guide for understanding the position and direction of the bone screw 1 in the medical image.

Note that at least a portion of the hole 20 is made of metal (e.g., titanium, stainless steel, iron, nickel, cobalt, cemented carbide, etc.), ceramic (e.g., zirconia), cermet, or reinforced fiber having a composition different from that of the reinforcing fiber 100. It suffices if it is surrounded by a material containing at least one fiber.

FIG. 8 is a diagram showing examples of the types of threads 5 of the bone screw 1. In the bone screw 1 shown in FIG. 8A, the flank angle of the thread 5 in the insertion direction is larger than the flank angle of the thread 5 in the removal direction. In the bone screw 1 shown in FIG. 8(b), the shaft portion 2 becomes thinner toward the insertion direction, and the diameter of the valley of the thread 5 becomes thinner and smaller toward the insertion direction. On the other hand, in the bone screw 1 shown in FIG. 8(b), the outer diameter of the thread 5 is substantially constant. In the bone screw 1 shown in FIG. 8(b), the height of the thread 5 increases toward the insertion direction. Further, in the bone screw 1 shown in FIG. 8(b), the length of the top of the screw thread 5 in the axial direction becomes smaller toward the insertion direction.

Due to the shape of the screw thread 5, the bone screw 1 can be reliably fixed to the bone and can be easily removed from the bone.

FIG. 9 is a diagram showing another example of the type of thread 5 of the bone screw 1. FIG. 9 is a cross-sectional view of the thread 5 of the bone screw 1. In the bone screw 1 shown in FIG. 9A, the flank angle α of the thread 5 in the insertion direction is larger than the flank angle β of the thread 5 in the removal direction. In the bone screw 1 shown in FIG. 9(b), the heights H1, H2, and H3 of the screw threads 5 (thread threads 5-1, 5-2, and 5-3) increase toward the insertion direction. In addition, in the bone screw 1 shown in FIG. 9(b), the axial lengths L1, L2, and L3 of the tops of the screw threads 5 (thread threads 5-1, 5-2, and 5-3) are in the insertion direction. The smaller it becomes. Note that the diameter of the valley of the thread 5 and the angle γ of the thread 5 are substantially constant. In the bone screw 1 shown in FIG. 9(c), the shaft portion 2 becomes thinner toward the insertion direction, and the diameter of the valley of the screw threads 5 (thread threads 5-4, 5-5, 5-6) is in the insertion direction. It becomes thinner and smaller as it goes towards the end. On the other hand, the outer diameter of the thread 5 and the angle γ of the thread 5 are substantially constant. The heights H4, H5, and H6 of the threads 5 (threads 5-4, 5-5, and 5-6) increase toward the insertion direction. In addition, in the bone screw 1 in FIG. 9(c), the axial lengths L4, L5, and L6 of the tops of the screw threads 5 (thread threads 5-4, 5-5, and 5-6) are in the insertion direction. The smaller it becomes.

FIG. 10 is a diagram showing that the bone screw 1 housed in the housing 6 is fixed within the housing 6 by a nut (not shown) and connected by a rod 7.

As shown in FIG. 10(a), the threads 5-7 of the bone screw 1 have the same shape as the threads 5 in FIG. 8(b). As shown in FIG. 10(b), the pitch of the threads 5 of the bone screw 1 becomes longer toward the insertion direction.

FIG. 11 is a CT image (medical image) taken with the bone screw 1 screwed into the bone of an animal (pig). In FIG. 11, the bone screw 1 contains tantalum of 0 weight % or more and 30 weight % or less (0 weight %, 10 weight %, 15 weight %, 20 weight %, 25 weight %, 30 weight %) and reinforcing fibers. including mixtures mixed into compositions.

FIG. 12 is a table evaluating artifacts of the bone screw 1 in CT images (medical images) taken according to the tantalum content shown in FIG. 11.

As shown in FIGS. 11 and 12, the higher the content of tantalum in the mixture, the stronger the artifact of the bone screw 1 in the CT image (medical image). When the content of tantalum in the mixture is 25% and 30% by weight, the artifact rating is A ^- , which is acceptable. Similar results are expected for other metals (titanium, stainless steel, iron, nickel, cobalt, cemented carbide, etc.). Therefore, the bone screw 1 (at least one of the shaft portion 2, the tip portion 3, the head 4, and the screw thread 5) contains more than 0% by weight and less than 30% by weight (preferably 10% by weight and less than 20% by weight), More preferably, the reinforcing fiber 100 contains a mixture of a metal (tantalum, titanium, stainless steel, iron, nickel, cobalt, cemented carbide, etc.) containing 10% by weight or more and 15% by weight or less, such as tantalum, titanium, stainless steel, iron, nickel, cobalt, and cemented carbide.

When the bone screw 1 (at least one of the shaft portion 2, the tip portion 3, the head 4, and the screw thread 5) is imaged with higher brightness than the surrounding area in a medical image while reducing artifacts, the bone screw 1 ( At least one of the shaft portion 2, the tip portion 3, the head portion 4, and the screw thread 5) functions as a guide for grasping the position and direction of the bone screw 1 in a medical image.

Additionally, the tip portion 3 may include a material that is imaged with higher brightness in medical images than the reinforcing fiber 100. For example, the tip portion 3 may be made of metal (e.g., tantalum, titanium, stainless steel, iron, nickel, cobalt, cemented carbide, etc.), ceramic (e.g., zirconia), cermet, or reinforcing fiber having a composition different from that of the reinforcing fiber 100. Contains at least one. When such a distal end 3 is imaged with higher brightness than the surrounding area of the distal end 3 in a medical image such as a CT image, the distal end 3 is used for grasping the position and direction of the bone screw 1 in the medical image. Act as a guide.

Although the embodiments according to the present invention have been described above, the present invention is not limited to these, and can be modified and modified within the scope of the claims.

The present invention is useful as a bone screw with sufficient strength.

1... Bone screw 2... Shaft part 3... Tip part 4... Head 5... Screw thread 6... Housing 7... Rod 20... Hole 21... Shaft recess 22... Notch 40... Recess 41... Head protrusion 41-1... Polygonal part 100...reinforced fiber

Claims

A bone screw containing reinforcing fibers,
a shaft part;
a tip portion located at the end of the shaft portion in the insertion direction;
a head located at the end of the shaft portion in the removal direction;
a screw thread located spirally in the axial direction of the shaft portion on the outer circumference of the shaft portion,
The bone screw characterized in that the reinforcing fibers are arranged in the shaft portion at a position extending in the axial direction of the shaft portion.
According to claim 1, there is a portion between the head portion and the shaft portion that has higher compressive strength, tensile strength, shear strength, torsional strength, and hardness than the shaft portion. Bone screw described.
The head is
a head convex portion in the insertion direction;
The bone screw according to claim 1 or 2, wherein the head convex portion fits into a shaft recess of the shaft portion.
At least one of the head convex portion and the shaft recess is a polygonal portion whose cross section perpendicular to the axial direction is a polygon, a thread located in a reverse helical direction to the thread, and at least one of a notch. The bone screw according to claim 3, comprising:
The bone screw according to claim 3, wherein the head protrusion includes at least one of metal, ceramic, cermet, and reinforcing fibers having a composition different from the reinforcing fibers.
The shaft portion is
A shaft convex portion is provided in the removal direction,
The bone screw according to claim 1 or 2, wherein the shaft protrusion fits into a head recess of the head.
At least one of the shaft convex portion and the head recess is at least one of a polygonal portion whose cross section perpendicular to the axial direction is a polygon, a screw thread located in a reverse helical direction with respect to the screw thread, and a notch. The bone screw according to claim 6, comprising:
comprising a core containing at least one of metal, ceramic, cermet, and reinforcing fibers having a composition different from the reinforcing fibers,
The bone screw according to claim 1 or 2, wherein the core part fits into a head recess of the head and a shaft recess of the shaft part.
9. The core includes the reinforcing fibers arranged in at least one of a position extending in the axial direction and a position spirally wound around the axial direction. bone screw.
comprising a hole that can penetrate the inside of the bone screw in the axial direction,
The bone screw according to claim 1 or 2, wherein the hole is surrounded by a material containing at least one of metal, ceramic, cermet, and reinforcing fibers having a composition different from the reinforcing fibers. .
The bone screw according to claim 1 or 2, wherein the tip portion includes a material that is imaged with higher brightness in medical images than the reinforcing fibers.
2. The reinforcing fiber is stretched in at least one of the shaft portion and the screw thread so as to be wound in a helical manner around the axial direction. 2. The bone screw according to 2.
The bone screw according to claim 1 or 2, wherein the reinforcing fibers are arranged in the screw thread at a position extending in the same direction as the helical direction of the screw thread.
The bone screw according to claim 1 or 2, wherein the reinforcing fibers are bent at the threads.
The bone screw according to claim 1 or 2, wherein the reinforcing fiber is at least one of carbon fiber, aramid fiber, and glass fiber.
At least one of the shaft portion, the tip portion, the head portion, and the screw thread includes a mixture in which more than 0% by weight and 30% by weight or less of metal is mixed into the composition containing the reinforcing fibers. The bone screw according to claim 1 or claim 2.
The bone screw according to claim 1 or 2, wherein the angle of the thread is 60 degrees or more and 80 degrees or less.
The bone screw according to claim 1 or 2, wherein the angle of the thread increases in the range of 60 degrees to 80 degrees as it goes toward the insertion direction.
The bone screw according to claim 1 or 2, wherein the flank angle of the thread on the insertion direction side is larger than the flank angle of the thread on the extraction direction side.
The bone screw according to claim 1 or 2, wherein the height of the thread increases toward the insertion direction.
The bone screw according to claim 1 or 2, wherein the length of the crest of the thread in the axial direction becomes smaller toward the insertion direction.
The bone screw according to claim 1 or 2, wherein the pitch of the thread becomes longer toward the insertion direction.